This reminds me of a hoax from the Yes Men [1]. They convinced temporarily the BBC that a company agreed to a compensation package for the victims of a chemical disaster, which resulted in a 4.23 percent decrease of the share price of the company. When it was revealed that it was a hoax, the share price returned to its initial price.

[1]: https://web.archive.org/web/20110305151306/http://articles.c...